In the PNW we very rarely use the seated back roll entry technique unless entering the water from a boat like a zodiac or similar. Usually we use a giant stride entry when entering from boats or docks. But the reason for the seated back roll is because when you're diving you have a lot of heavy and loose gear, and you want to be in control of it all and yourself the whole time. The seated back roll (SBR from now on) is really good for wrangling your hoses and accessories in front of you where it can all be held onto while at the same time holding the clasp of your weight belt. It also allows you to lead with the hardest heaviest and least fragile part of your body and gear: the tank. If say, a log floated up next to the boat while you were managing all your gear and getting ready to take a dip, and you didn't see it, when you roll backwards, your tank shields you from the blow of hitting the log from a small height with sometimes nearly 100 lbs of gear on top of you. It also serves to propeller you away from the boat quickly and with little effort. This way, you clear the boat, which may be bobbing up and down on rough seas, as well as allow someone else to enter behind you. It also serves to, usually, orient you facing the boat when you surface so you can signal the boat that you're OK and do not need assistance. Also, if the seas are you specially rough, your tank breaks the water for you so you don't get smacked in the face with an upswell right as the boat breaks the crest of the previous wave and you fall face first into it (no fun).

Tl;Dr - it's easier and safer than the alternatives sometimes. Source - am PADI Divemaster, out of teaching status Master SCUBA Diver Trainer. Yes, I know, we have the best "ranks/titles"